Narset, Enlightened Exile

If a creature has multiple instances of prowess, each triggers separately.

Narset's first ability gives it prowess while it's on the battlefield.

If another effect causes Narset's power to be less than or equal to the mana value of the target card as the ability tries to resolve, the target is illegal. The card won't be exiled, and you won't get to cast a copy.

If you don't want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are performed.